Assessment of Soil Health and Preparation of Soil Health Card

Assessment of Soil Health and Preparation of Soil Health Card

Sanjay Srivastava Pramod Jha A. K. Tripathi Pradip Dey Ashok K. Patra

ICAR-Indian Institute of Soil Science Nabi Bagh, Berasia Road, Bhopal - 462038 (M.P.)

Printed: January, 2015

Assessment of Soil Health and Preparation of Soil Health Card

Sanjay Srivastava Pramod Jha A. K. Tripathi Pradip Dey Ashok K. Patra

All Rights Reserved 2015, ICAR-Indian Institute of Soil Science, Bhopal

Published by the Director, ICAR-Indian Institute of Soil Science, Nabi Bagh, Berasia Road, Bhopal - 462 038.

Contents

Foreword Preface Prologue 1.

Soil Health and soil health card

2.

In-field assessment of soil health

3.

Quantitative soil health assessment

4.

Soil testing and soil sample collection

5.

Appendix I: An example of soil health card

6.

Appendix II: Rating of soils as per soil health indicators

7.

Appendix III: Analytical methods for analysis of soils

8.

Appendix IV: Some other information

9.

Literature consulted

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Foreword The major challenges in 21st century are food security, environmental quality and soil health. Besides, shrinking land holdings and increasing cost of inputs in India merit adoption of scientific use of plant nutrient for higher crop productivity. For rational use of plant nutrient, assessment of soil health is of paramount importance. It is a matter of great pleasure for me to find this soil health manual prepared by ICAR-Indian Institute of Soil Science, Bhopal. What more can be more appropriate and timely than this soil health manual in the light of 2015 being declared by United Nations as “International Year of Soil”. This manual is even more important in the light of a nationwide programme of the Government of India of distribution of soil health cards to every farmer of the country. Indeed soil is the soul of infinite life and no nation can prosper on sustainable basis without giving due consideration to the health of its soil.

This manual, interalia, contains the basic methodology of preparing a soil health card. The interpretation and advisory is also included. This will prove useful for researchers, policymakers, and soil health assessment personnel in preparing the soil health cards of their respective states. I congratulate the editors for their efforts in bringing this invaluable publication.

( S. Ayyappan )

i

Preface

To be written

(Ashok K. Patra)

ii

Prologue It has been long felt that there is a need of a manual on the preparation of soil health card. We have different types of soil health indicators and soil health card many of which are more comprehensive than the one presented here. This manual has been compiled from various resources and customized to be used under Indian conditions. Still, it is indicative in nature. The actual soil health cards may differ from region to region depending upon the local conditions and requirement. We have given two types of soil health assessment, one being largely qualitative in nature and to be examined in field. The other is quantitative assessment of soil health which requires laboratory infrastructure.

It may be worthwhile to mention that approximately 140 million fields will have to be assessed if the soil health card is prepared for whole of the country (assuming one representative sample from one hectare of land). This is a gigantic task and would require about 4666 laboratories, assuming one third of the samples are analyzed in one year and capacity of one laboratory to analyze samples in one year is 10000 samples. Obviously, this number of laboratories is far more than the existing number of soil testing laboratories in India which is less than 1100. Hence, one has to find new ways and means to accomplish the task. One alternative would be the development of new soil test kits/mini labs will have to be evolved that would give a quantitative assessment of important soil fertility indicators. An automation would also be required in the collection of soil samples as the time to collect the soil samples is not more than three months.

Soil health is looked into with respect to physical, chemical, and biological indicators and all have been incorporated in this manual.

However, one of the most important reasons of assessing the health of a particular soil/field is to have a fair idea about its future management, most importantly the fertilizer/manure management. This aspect is especially important for India because a huge amount of foreign exchange is spent every year on import of fertilizers. Potassic fertilizers are entirely imported and almost 90% of the phosphatic fertilizers are imported either as finished product or in the form of raw materials like rock phosphate, phosphoric acid, and sulphur. Though urea is manufactured in India, still a large part of it is imported. Hence, soil fertility management and soil test based balanced fertilizer applications are most important aspects in the assessment of soil health. Hence, more emphasis is given in this publication on soil fertility assessment. According to one study, the percent samples falling under deficient category on pan Indian scale 89% for N, 80% for P, 50% for K, 49% for Zn, 41% for S, 33% for B, 13% for Mo, 12% for Fe, 5% for Mn, and 3% for Cu. Since the deficiency of S, Zn, and B are widespread, the parameters that may need to be analyzed for almost all the soils are pH, EC, Organic carbon, available N, P, K, S, Zn, and B. Some other nutrients like Fe, Mo, Ca, and Mg may need to be assessed under some specific situations. Besides, the soils having pH more than 8.5 and/or EC more than 4 mmhos/cm would require an assessment of

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gypsum requirement and analysis of irrigation water. Similarly, the soils having pH below 5.5 would have to be analyzed for lime requirement.

Similarly, a more comprehensive account of deficiency/toxicity of micronutrients/heavy metals is available in the annual reports of All India Coordinated Research Project on Micronutrients, the coordinating unit of which is located at ICAR-IISS, Bhopal. Also, the interpretation of soil test values of primary nutrients for fertilizer advisory purposes can be done based on the technologies of soil test based fertilizer requirement for targeted yields of different crops, available at All India Coordinated Research Project on Soil Test Crop response Correlations, the headquarter of which is also located at ICAR-IISS, Bhopal. The know-how available at various Agricultural Universities can also be utilized.

Some of the qualitative soil health indicators, proposed to be judged in field/lab may require training and experience. For example, the texture assessment by feel method requires experience of working with soils of known texture. Similarly, an assessment of soil compaction would require training. These, and some other parameters like slope assessment of the field, and penetrometer resistance may be skipped, especially on already cultivated lands, if the proper paraphernalia and experience is not available. The local knowledge of the farm owner is very important and that must be taken into consideration while preparing the soil health card.

This soil health assessment manual is mainly prepared taking into consideration of soil health indicators for agricultural purposes. A related activity is to prepare soil fertility maps. Hence, we recommend that, wherever possible, the GPS coordinates of the representative samples may be recorded. This would also help to study the periodic changes in soil fertility parameters.

Finally, we reiterate that this soil health manual and proposed soil health card is indicative in nature. We have found several formats of soil health card viz., soil health card of TNAU, Coimbatore, HAU, Hisar, Gujarat state and others and they are different than the one mentioned in this publication. We leave on the discretion of various state governments to prepare their own soil health cards using their own resources pooling the soil test laboratories, soil test kits, minilabs etc. Of course, a consultation with concerned Ag. University is essential. When a soil test kit is to be used, it has to be well calibrated against the standard methods of estimation of soil parameters. Some Minilabs could also be developed which could give quantitative/semiquantitative results of the soil parameters.

Editors

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Soil Health Soil Health is the capacity of the soil to function to sustain life. A healthy soil can be used productively without adversely affecting its future productivity, the ecosystem or the environment. Soil health emphasizes the integration of biological with chemical and physical measures of soil quality (used synonymously with “soil health�) that affect farmers' profits, risks, and the environment.

Government of India has launched a scheme to provide every farmer a Soil Health Card in a Mission mode. The card will carry crop wise recommendations of nutrients/fertilizers required for farms, making it possible for farmers to improve productivity by using appropriate inputs.

A healthy soil has the following characteristics (Adapted from Cornell Soil Health Assessment Training Manual): 1. Good soil tilth: Soil tilth refers to the overall physical character of the soil in the context of its suitability for crop production.

2. Sufficient depth: Sufficient depth refers to the extent of the soil profile to which roots are able to grow and function. A soil with a shallow depth is more susceptible to extreme fluctuations in the weather, thus making the crop susceptible to drought or flooding stress.

3. Sufficient but not excess supply of nutrients: An adequate and accessible supply of nutrients is necessary for optimal plant growth. Excess nutrients can lead to leaching and potential ground water pollution, high nutrient runoff and greenhouse gas losses, as well as toxicity to plants and microbial communities.

4. Plant pathogens and insect pests: In agricultural production plant pathogens and pests can cause diseases and damage to the crop. In a healthy soil, the population of these organisms is low and/or inactive.

5. Good soil drainage: A healthy soil will drain more rapidly as a result of good soil structure and an adequate distribution of different size pore spaces, but also retain adequate water for plant uptake.

6. Large population of beneficial organisms: A healthy soil will have a high and diverse population of beneficial organisms to carry out these functions and thus helps to maintain a healthy soil status.

7. Low weed pressure: A healthy soil should have minimal amount of weeds. Weed pressure is a major constraint in crop production. Weeds compete with crops for water and nutrients that are essential for plant growth.

8. Free of chemicals and toxins that may harm the crop: Healthy soils are either devoid of harmful chemicals and toxins or can detoxify and/or bind such chemicals making them unavailable for plant uptake due to their richness in stable organic matter and diverse microbial communities.

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9. Resistant to degradation: A healthy, well aggregated soil is more resistant to adverse events including erosion by wind and rain, excess rainfall, extreme drought, vehicle compaction, etc.

10. Resilience when unfavorable conditions occur: A healthy soil will rebound more quickly after a negative event such as harvesting under wet soil conditions or if land constraints restrict or modify planned rotations.

Soil Health Card A Soil Health Card contains information on the current status of soil health and, when used over time, can be used to determine changes in soil health that are affected by land management. A Soil Health Card displays soil health indicators and associated descriptive terms. This Card is a tool to help the farmers, extension functionary, researcher and decision makers to monitor and improve the soil health based on their own field experience and working knowledge to maximize the production with scientifically justified use of fertilizers. Soil health needs to be assessed at regular intervals so as to ensure that farmers apply the required nutrients while taking advantages of the nutrients already present in the soil. Regular practices of soil testing will enable the farmers and other related functionaries to record long-term trends in soil health and to assess the effects of different soil management practices, responses of application of nutrients, crop selection etc., which intern facilitate to draw indicators that assess each soil's ability to support crop production within its capabilities and field limitations.

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In-field Assessment of Soil Health (Optional) Before laboratory estimation is made it is advisable to make a field assessment of soil. This does not require any sophisticated instruments. This is, however, qualitative in nature and should be supported by laboratory assessment. The soil quality indicators that can be assessed in field are given in the table below.

In field soil quality assessment Indicator

Best time of taking observation

Soil colour (indicative of organic In moist soil carbon/humus)

Soil health Good

Medium

Poor

Top soil darker than subsoil

Surface colour closer to subsoil colour

Topsoil colour similar to subsoil colour

Earthworms

Rainy season under good soil moisture

> 10 worms per 0.092 cubic meter, lots of casts and holes visible

2-10 worms per 0.092 cubic meter, casts or holes visible

0-1 per 0.092 cubic meter, cast or holes not visible

Organic matter roots/residue

Anytime

Prominent roots and residues visible

Few roots

Roots or residues not visible

Compaction

To be taken at field capacity, usually several days after free drainage, best pretillage or post harvest

Wire goes easily with fingers up to 30 cm

Have to push hard with fist

Wire breaks or bends while inserting.

Soil tilth

Under good soil moisture

Having spheroidal or crumb structure

Cloddy

Looks dead, like brick or concrete, hard to pull drill through

Water holding capacity

After rainfall during growing season

Holds water for a long period of time.

Water runs out after a week or so.

Plant stress two days after a good rain.

Drainage/infiltration

After rainfall

No ponding, no run off, water drains steadily, soil not too wet not too dry.

Water remains for a short period of time, eventually drains.

Water evaporates more than it drains, always very wet field.

Soil texture (Feel method).

Anytime

Loamy

See appendix for interpretation

Quantitative Soil Health Assessment Following soil health indicators may be assessed quantitatively Soil health indicators Physical

Chemical

Biological

Depth of the soil: Dig the soil with a spade to Soil Chemical Composition: a

Organic carbon: It is any material that is

find the depth of hard pan. See appendix for standard soil test analysis package

derived from living organisms, including

procedure.

measures levels of pH, EC, and

plants, soil fauna, and added organic matter.

available soil nutrients. Measured

Total soil organic matter consists of both

levels are interpreted in the

living and dead material, including well

framework of low, medium, or high

decomposed humus.

Surface Hardness: It is a measure of the maximum soil surface (0 to 15 cm depth) penetration resistance (psi) determined using a field penetrometer.

for primary nutrients and critical level for secondary and micronutrients.

Subsurface hardness: It is a measure of the Optional: Under some situations maximum resistance (in psi) encountered in toxicity of some chemicals need to the soil at the 15 to 45 cm depth using a field be estimated. penetrometer.

The percent Organic C is determined by wet digestion procedure (Walkley and Black, 1934).

Soil Testing and Soil Sample Collection This is a process by which elements (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, manganese, copper, iron, zinc, boron, and molybdenum) are chemically removed from the soil and measured for their "plant available" content within the sample.

The quantity of available nutrients in the sample determines the amount of fertilizer that is recommended. A soil test also measures soil organic carbon, soil pH, and electrical conductivity (EC). However, soil tests may also include the determination of alkalinity and acidity. The most important step in soil testing is the collection of a representative soil sample.

Soil Sampling Protocol Soil sampling is one of the most important steps in any soil testing programme. A non representative sample may vitiate the entire results. Only few grams of soil samples are tested for different parameters in the laboratory. This small amount of soil gives indication of that particular nutrient for the whole field which weighs around 2.24 x 106 kg (one hectare furrow slice) the entire soil.

Hence, the samples should be accurately taken. Some precautions that need to be followed are: 1. Do not collect the samples at the periphery. There is chance of contamination as fertilizer bags are kept their, animals and other humans pass through the periphery.

2. When collecting the samples first demarcate the area for each subsample based on uniformity in colour, slope, drainage, and contrasting past management practices.

3. Never collect the samples in line. Instead, sampling at several locations in a zig-zag pattern ensures homogeneity.

4. Avoid sampling in dead furrows, wet spots, areas near main bund, trees, manure heaps and irrigation channels.

5. For shallow rooted crops, collect samples up to 15-20 cm depth. For deep rooted crops, collect samples up to 30 cm depth.

For tree crops, collect profile samples.

6. Clean the spot before taking the soil sample. Remove the surface litter at the sampling spot.

7. It is very important to collect a uniform slice up to sampling depth. It is a common mistake done during sampling when the person digging the hole keeps on collecting the sample while digging. Under such case the sampling becomes skewed with respect to depth. When using khurpi, the recommended procedure is to first dig a V shaped cut up to a plough layer (15-20 cm). Remove all the soil. Then take a uniform 1.5-2.5 cm thick slice from the side (See figure below).

8. At least 10-15 slices should be collected to get a representative sample. The demarcation of area for taking a representative sample is to be done based on visual inspection and past experience regarding the uniformity of soil. The sampling intensity may be increased for larger fields. In case a representative soil sample is to be collected for larger fields (say 10 hectare) care has to be taken in drawing the sub-samples from a uniform area as discussed in S. No. 2 above.

9. Generally higher spatial variability is encountered in irrigated than rain-fed situations. Hence, sampling intensity may be increased in irrigated conditions. Depending upon the purpose and availability of resources under these two different 5

conditions the sampling unit may go up-to 2.5 ha in irrigated and 10 ha in rain-fed condition. However, the points mentioned in S. No. 2 for drawing sub-samples have to be followed.

10. Always collect the soil sample in presence of the farm owner who knows the farm better.

11. Collect the sub samples in a plastic tray.

15-20 cm

1.5-2.5 cm Method of taking soil sample

Sampling frequency and timing: Soil testing should be carried out once in every three to five years. Take the soil sample well before sowing/planting, so there is time to treat the soil. Sampling should be done at the same time each year. It is better to take the samples in the same month of the year in which the previous samples were taken. In India samples can be taken during April to June but prior to the application of manures/fertilizers.

Depth: A sampling depth of 15-20 cm is required for most of the field crops. For a pasture crop, a 10 cm depth is normally sufficient.

However, for deep rooted crops like sugarcane, cotton, and horticultural crops, sampling from different depths may be needed.

Implement: Khurpi, tube auger or spade may be used for sampling of soft or moist soils. A screw auger can be used on hard or dry soils.

After Collection of Sub Samples 

Mix the samples thoroughly and remove foreign materials like roots, stones, pebbles and gravels.



Reduce the bulk to about half to one kilogram by quartering or compartmentalization.



Quartering is done by dividing the thoroughly mixed sample into four equal parts. The two opposite quarters are discarded and the remaining two quarters are remixed and the process repeated until the desired sample size is obtained.

Compartmentalization is done by uniformly spreading the soil over a clean hard surface and dividing into smaller compartments by drawing lines along and across the length and breadth. From each compartment a pinch of soil is collected. This process is repeated till the desired quantity of sample is obtained (see figure below).



Collect the sample in a clean cloth or polythene bag.



Label the bag with information like name of the farmer, location of the farm, survey number, previous crop grown, present crop, crop to be grown in the next season, date of collection, name of the sampler etc.

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1

13 A schematic diagram showing soil sample locations

Reject

Use

Use

Reject

Quartering technique for reducing the soil volume before analysis

Processing and Storage 1. Assign the sample number and enter it in the laboratory soil sample register.

2. Dry the sample collected from the field in shade by spreading on a clean sheet of paper after breaking the large lumps, if present. The samples should be dried at 25-35ď‚° C. 3. The soil aggregates should be broken up by crushing lightly wooden roller, wooden pestle and mortar.

4. Sieve the soil material through 2 mm stainless steel or plastic sieve.

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5. Repeat powdering and sieving until only materials of >2 mm (no soil or clod) are left on the sieve.

6. Collect the material passing through the sieve and store in a clean glass or plastic container or polythene bag with proper labeling for laboratory analysis. (If the samples are meant for the analysis of micronutrients, care is needed in handling the sample to avoid contamination of iron, zinc and copper. Brass or iron sieves should be avoided and it is better to use stainless steel or polythene materials for collection, processing and storage of samples.)

7. For the determination of organic carbon it is desirable to grind a representative sub sample and sieve it through 0.5 mm sieve.

8. Air-drying of soils must be avoided if the samples are to be analyzed for NO3-N and NH4-N as well as for bacterial count.

9. Field moisture content must be estimated in un-dried sample or to be preserved in a sealed polythene bag immediately after collection.

10. Estimate the moisture content of sample before every analysis to express the results on dry weight basis.

11. Avoid contact of the sample with chemicals, fertilizers or manures.

12. Use stainless steel augers instead of rusted iron khurpi or kassi for sampling for micronutrient analysis.

13. Do not use bags or boxes previously used for storing fertilizers, salt or other chemicals. Store soil sample in clean, preferably new, cloth or polythene bags. Use glass, porcelain or polythene jar for long duration storage.

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Appendix I An Example of Soil Health Card Name: S/o: Address: Date of field inspection

Field identification: GPS Coordinates (Optional): Area of the field: Date of collection of samples

General Information Land type: Cultivated/Non-cultivated Irrigation: Irrigated/non irrigated Terrain: Flat/undulating Slope (Optional): <15% 15-30% > 30% No. of animals with the farmer Cows Buffalos Goat Whether organic manure available with the farmer?

Others

Field Inspection Report Indicators Organic matter colour Earthworms Organic matter roots/residue Compaction Soil tilth Water holding capacity Drainage/infiltration

Observation

Rating

Quantitative Results Physical Depth of the soil

Results Less than 50 cm 50 to 90 cm More than 90 cm

Rating Poor Fair Good

Surface hardness

< 30% of measuring points having cone Little/no compaction index > 300 psi 30 â&#x20AC;&#x201C; 50% Slight compaction 50-75% Moderate compaction > 75%

Subsurface hardness Soil Texture

Severe compaction

Similar interpretation as above.

Qualitative (See appendix) 9

Chemical

Result

Rating Good

Fair

Poor

Soil pH

Neutral Slightly acid

Soil EC

Salt free

Moderately acid/Moderately alkaline Slightly saline

See appendix II for interpretation of results

Soil test levels are in high category or above critical level.

No visible signs of plant nutrient deficiency or toxicity.

One or more soil test levels are Medium or low category.

No visible signs of plant nutrient deficiency.

Extremely acid to strongly acidic /Strongly alkaline Moderately saline to highly saline One or more soil test levels are deficient/low or excessive for crop growth.

Visible signs of plant nutrient deficiency toxicity.

Result

Rating Good > 0.75%

Fair 0.5-0.75%

Available N Available P Available K Secondary Nutrient (S, Ca (optional), and Mg (optional) Available micronutrients (Fe (optional), Zn, Cu (optional), Mn (optional), B, Mo (optional))

Biological Organic carbon

Fertilizer recommendation

Poor < 0.5%

Crop (To be specified) (wheat/rice/maize/sugarcane/cotton/others)

N (kg/ha) P (kg/ha) K (kg/ha) S (kg/ha) Micronutrient (if required) Amendment (Gypsum/pyrite/lime), if required: Organic manure

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Appendix II Rating of Soils as Per Soil Health Indicators Rating Chart for Soil Test Values of Primary Nutrients Nutrient

Rating Low ≤ 0.5 ≤ 280 ≤ 10 ≤ 120

Organic carbon (%) Alkaline KMnO4-N (kg/ha) Olsen‟s P (kg/ha) Amm. Acetate-K (kg/ha)

Medium 0.51-0.75 281-560 10.1-25 121-280

High > 0.75 > 560 > 25 > 280

Rating Chart for Soil Test Values of DTPA Extractable Micronutrients Soil Test Rating Critical level*

Zinc Iron ----------------------------------- ppm ----------------------------------0.6 (0.4 – 1.2) 4.5

Manganese

Copper

2.0

0.2

Rating Chart for Soil Test Values of Other Nutrients Soil Test Rating

0.15% CaCl2 extractable S (Williams and Steinbergs 1959)

Calcium Ammonium acetate method

Magnesium Ammonium acetate method

Hot water soluble boron

Molybdenum (Grigg’s reagent)

Critical* level

10 (8-30 ppm)

<25% of CEC or < 1.5 me Ca/100 g

<4% of CEC or <1 me Mg/100g

0.5

0.2 For alkaline soils: 0.05

* The critical level or a critical limit is that level of a nutrient in soil which is likely to result 90% of the maximum yield. The values below the critical limits indicate the need for fertilization of that nutrient.

These critical limits are only indicative in nature. The users should consult the Agricultural Universities of the respective states for the exact values of critical limits.

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Rating Chart for Soil Ph Soil pH Value

Soil Reaction

Rating

> 8.5

Strongly alkaline

Poor

7.1 to 8.5

Moderately alkaline

Fair

7.0

Neutral

Good

6.6 to 6.9

Slightly acid, maximum availability of all the essential plant nutrients

Good

5.6 to 6.5

Moderately acid

Fair

4.6 to 5.5

Strongly acid

Poor

3.5 to 4.6

Extremely acid soils of warm to humid and high rain fall areas. (Laterite soils)

Poor

Less than 3.5

Acid Sulphate soils (Kerala costal belt)

Poor

Rating Chart for Soil EC EC (mS/cm)

Salt content of soil

Crop reaction

0-2

Low

Good: Salinity effect negligible, except for more sensitive crops

2-4

Moderate

Fair: Yield of many crops restricted

4-8

High

Most non-salt tolerant plants will show injury; salt-sensitive plants will show severe injury

8-16

Excessive

Poor: Salt-tolerant plants will grow; most others show severe injury.

>16

Very excessive

Poor: Very few plants will tolerate and grow

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Appendix III Analytical Methods for Analysis of Soils 1. Soil Moisture Gravimetric method of moisture estimation is most widely used where the soil sample is placed in an oven at 1050 C and dried to a constant weight. The difference in weight is considered to be water present in the soil sample.

Apparatus required Aluminium Moisture Box, Oven, Desiccator Procedure 1. Take 100 g of soil sample in the aluminium moisture box and keep in the oven after removing the lid of the box.

2. The sample is kept at 1050 C till it attains a constant weight. It may take 24-36 hours.

3. Cool the sample, first in the switched off oven and then in a desiccator.

4. Take the weight of the cooled moisture box. The loss in weight is equal to moisture contained in 100 g soil sample.

Calculation Moisture percentage = [(Loss in weight)/(over dry weight of soil)] x 100 The corresponding moisture correction factor (mcf) for analytical results or the multiplication factor for the amount of sample to be weighed in for analysis is: Moisture correction factor = (100 + % moisture)/100

2. pH The soil pH is the negative logarithm of the active hydrogen ion (H+ ) conc. in the soil solution. It is the measure of soil sodicity, acidity or neutrality. It is a simple but very important estimation for soils, since soil pH influences to a great extent the availability of nutrients to crops. It also affects microbial population in soils.

The procedure for measurement of soil pH is given below.

Apparatus • • • •

pH meter with a range of 0-14 pH Pipette/dispenser Beaker Glass rod

Reagent 13

• •

Buffer solutions of pH 4, 7 and 9 Calcium chloride solution (0.01M): Dissolve 14.7 g CaCl2.2H2O in 10 litre of water to obtain 0.01M solution.

Procedure 1) Calibrate the pH meter, using 2 buffer solutions, one should be the buffer with neutral pH (7.0) and the other should be chosen based on the range of pH in the soil. Take the buffer solution in the beaker. Insert the electrode alternately in the beakers containing 2 buffer solutions and adjust the pH. The instrument indicating pH as per the buffers is ready to test the samples 2.

2) Weigh10.0g of soil sample into 50 or 100 ml beaker, add 20ml of CaCl2 solution (use water instead of CaCl2 solution throughout the procedure if water is used as a suspension medium).

3) Allow the soil to absorb CaCl2 solution without stirring, then thoroughly stir for 10 second using a glass rod.

4) Stir the suspension for 30 minutes and record the pH on the calibrated pH meter.

The acidic soils need to be limed before they can be put to normal agricultural production. The alkali soils need to be treated with gypsum to remove the excessive content of sodium.

(2 A) Lime Requirement Crop yields are normally high in soils with pH values between 6.0 and 7.5. Lime is added to raise the pH of acid soils, and the amount of lime required to raise the pH to an optimum level is called as Lime Requirement. A number of methods are available for the determination of lime requirement. The Woodruff and the Shoemaker et al methods are discussed here which are based on the use of a buffer solution, whose pH undergoes change when treated with acid soils. The pH of buffer solution will gradually decrease when H+ ion concentration increases. When H+ increases by 1 meq in 100 ml buffer solution, pH value will decrease by 0.1 unit. Buffer solutions needs to be prepared afresh. A 0.05M solution of AR grade potassium hydrogen pthalate (molecular weight 204.22) gives a pH of 4.0 at 250 C and it can be used as a buffer.

Woodruff’s Method (Woodruff, 1948) Apparatus • pH meter • Automatic pipettes Reagent • Woodruff‟s buffer solution: Dissolve 10 g calcium acetate [Ca(CH3COOH)2], 12 g para-nitrophenol, 10 g salicylic acid and 1.2 g sodium hydroxide in distilled water. Adjust pH to 7.0 with acetic acid or sodium hydroxide, transfer to 1 litre volumetric flask and make the volume to the mark with distilled water.

Procedure 1. Take 10 g soil sample in a clean 50 ml beaker. 2. Add 10 ml of distilled water, stir and wait for 30 minutes. 3. Determine pH value in soil suspension. 4. If the pH value is less than 5.0 (average of 4.5 and 5.5 to have one value), add 10 ml Woodruff‟s buffer solution, stir and wait for 30 minutes before determining new pH value. The amount of lime required to raise the pH for agricultural purpose is shown in Table below: Table: pH and quantity of lime required to reduce soil acidity (tonnes/ha) 14

pH (after buffer) 6.5 6.4 6.3 6.2 6.1 6.0 5.9 5.8 5.7 5.6

CaCO3 6 7.2 8.4 9.6 10.8 12 13.2 14.4 15.6 16.8

Ca(OH)2 4.68 5.62 6.55 7.49 8.42 9.36 10.3 11.23 12.17 13.1

Marl 7.2 8.64 10.08 11.52 12.96 14.4 15.84 17.28 18.72 20.16

Limestone 9 10.8 12.6 14.4 16.2 18 19.8 21.6 23.4 25.2

Dolomite 6.54 7.85 9.16 10.46 11.77 13.08 14.39 15.7 17 18.31

The soils between pH 6.6 and 7.5 are practically considered as nearly neutral. Such soils do not need to be treated with lime or gypsum. Even in case of soils, which are acidic and alkaline beyond these limits, growing of acid loving and salt tolerant crops may be considered. Only highly acidic soils and the soils with high alkalinity need to be treated with chemical amendments since this operation is quite expensive.

Shoemaker Method (Shoemaker et al., 1961) Apparatus • pH meter • Automatic pipettes - 10 and 20 ml Reagent • Extractant Buffer: Dissolve 1.8 g of nitrophenol, 2.5 ml triethanolamine, 3.0 g potassium chromate (K2CrO4), 2.0 g calcium acetate and 53.1 g calcium chloride in a litre of water. Adjust pH to 7.5 with NaOH. Procedure 1. Take 5.0 g soil sample in a 50 ml beaker.

2. Add 5 ml distilled water and 10 ml extractant buffer.

3. Shake continuously for 10 minutes or intermittently for 20 minutes and read the pH of the soil buffer suspension with glass electrode. The pH of the buffer solution is reduced, depending upon the extent of soil acidity.

For various levels of measured pH of soil buffer suspesion, the amount of lime required to raise the soil pH to 6.0, 6.4 and 6.8 is given in Table below in terms of CaCO3. The decrease of buffer pH by 0.1 unit is equivalent to 1 meq of H+ in 100 ml buffer solution.

The lime requirement varies with the type of soils and their cation exchange capacity.

Measurement of pH soil buffer Lime Requirement in tonnes/ha as CaCO3 for bringing suspension soil pH to different levels 6 2.43 3.4

6.7 6.6 15

6.4 2.92 4.13

6.8 3.4 4.62

6.5 6.4 6.3 6.2 6.1 6.0

4.37 5.59 6.65 7.52 8.5 9.48

5.35 6.56 7.78 8.93 10.21 11.42

6.07 7.53 8.99 10.21 11.66 13.12

Practically, pH of acid soils may not be raised beyond 6.4/6.5.

3. EC The electrical conductivity (EC) is a measure of the ionic transport in a solution between the anode and cathode. This means, the EC is normally considered to be a measurement of the dissolved salts in a solution. Like a metallic conductor, they obey Ohm‟s law. Since the EC depends on the number of ions in the solution, it is important to know the soil/water ratio used.

Apparatus • EC meter • Beakers (25 ml), Erlenmeyer flasks (250 ml) and pipettes • Filter paper Reagent • 0.01M Potassium chloride solution: Dry a small quantity of AR grade potassium chloride at 600C for two hours. Weight 0.7456 g of it and dissolve in freshly prepared distilled water and make the volume to one litre. This solution gives an electrical conductivity of 1411.8x10-3 i.e. 1.412 mS/cm at 250C. For best result, select a conductivity standard (KCl solution) close to the sample value.

Procedure 1. Take 40 g soil into 250 ml Erlenmeyer flask, add 80 ml of distilled water, stopper the flask and shake on reciprocating shaker for one hour. Filter through Whatman No.1 filter paper. The filtrate is ready for measurement of conductivity.

2. Wash the conductivity electrode with distilled water and rinse with standard KCl solution.

3. Pour some KCl solution into a 25 ml beaker and dip the electrode in the solution. Adjust the conductivity meter to read 1.412 mS/cm, corrected to 250 C. 4. Wash the electrode and dip it in the soil extract.

5. Record the digital display corrected to 250 C. The reading in mS/cm of electrical conductivity is a measure of the soluble salt content in the extract, and an indication of salinity status of this soil. The conductivity can also be expressed as mmhos/cm.

(3 A) Gypsum requirement (Schoonover, 1952) In the estimation of gypsum requirement of saline-sodic/sodic soils, the attempt is to measure the quantity of gypsum (Calcium sulphate) required to replace the sodium from the exchange complex. The sodium so replaced with calcium of gypsum is removed through leaching of the soil. The soils treated with gypsum become dominated with calcium in the exchange complex.

When Calcium of the gypsum is exchanged with sodium, there is reduction in the calcium concentration in the solution. The quantity of calcium reduced is equivalent to the calcium exchanged with sodium. It is equivalent to gypsum requirement of the soil when „Ca‟ is expressed as CaSO4.

16

Apparatus • Mechanical shaker • Burette – 50 ml • Pipettes – 100 ml and 5 ml Reagents •

Saturated gypsum (calcium sulphate) solution: Add 5 g of chemically pure CaSO4.2H2O to one litre of distilled water. Shake vigorously for 10 minutes using a mechanical shaker and filter through Whatman No.1 filter paper.

• 0.01N CaCl2 solution: Dissolve exactly 0.5 g of AR grade CaCO3 powder in about 10 ml of 1:3 diluted HCl. When completely dissolved, transfer to 1 litre volumetric flask and dilute to the mark with distilled water. CaCl2 salt should not be used as it is highly hygroscopic.

• 0.01N Versenate solution: Dissolve 2.0 g of pure EDTA – disodium salt and 0.05 g of magnesium chloride (AR grade) in about 50 ml of water and dilute to 1 litre. Titrate a portion of this against 0.01N of CaCl2 solution to standardize.

• Eriochrome Black T (EBT) indicator: Dissolve 0.5 g of EBT dye and 4.5 g of hydroxylamine hydrochloride in 100 ml of 95% ethanol. Store in a stoppered bottle or flask.

• Ammonium hydroxide-ammonium chloride buffer: Dissolve 67.5 g of pure ammonium chloride in 570 ml of conc. ammonium hydroxide and dilute to 1 litre. Adjust the pH at 10 using dilute HCl or dilute NH4OH. Procedure 1.

2.

3.

4.

5.

6.

Weigh 5 g of air-dry soil in 250 ml conical flask.

Add 100 ml of the saturated gypsum solution. Firmly put a rubber stopper and shake for 5 minutes.

Filter the contents through Whatman No.1 filter paper.

Transfer 5 ml aliquot of the clear filtrate into a 100 or 150 ml porcelain dish.

Add 1 ml of the ammonium hydroxide-ammonium chloride buffer solution and 2 to 3 drops of Eriochrome black T indicator.

Take 0.01N versenate solution in a 50 ml burette and titrate the contents in the dish until the wine red colour starts changing to sky blue. Volume of versenate used = B. 7. Run a blank using 5 ml of saturated gypsum solution in place of sample aliquot. Volume of versenate solution used = A. Calculation Gypsum requirement (tonnes/ha) = (A – B) x N x 382 where, A = ml of EDTA (versenate) used for blank titration B = ml of ETDA used for soil extract N = Normality of EDTA solution

4. Mineralizable (Available) Nitrogen (Subbiah And Asija, 1956) In case of soils, mineralizable N (also organic C) is estimated as an index of available nitrogen content and not the total nitrogen content. The easily mineralizable nitrogen is estimated using alkaline KMnO4, which oxidizes and hydrolyses the organic matter present in the soil. The liberated ammonia is condensed and absorbed in boric acid, which is titrated against standard acid. The method has been widely adopted to get a reliable index of nitrogen availability in soil due to its rapidity and reproducibility. The process of oxidative hydrolysis is, however, a progressive one and thus, a uniform time and heating temperature should be allowed for best results. Use of glass beads checks bumping while liquid paraffin checks frothing during heating as is recommended in total N estimation by Kjeldahl method.

Apparatus 17

• .Nitrogen distillation unit, preferably with six regulating heating elements.

• Conical flasks, pipettes, burette, etc.

Reagents • • • •

0.32% KMnO4: Dissolve 3.2 g of KMnO4 in water and make the volume to one litre.

2.5% NaOH: Dissolve 25 g of sodium hydroxide pellets in water and make the volume to one litre.

2% Boric acid: Dissolve 20 g of boric acid powder in warm water by stirring and dilute to one litre.

Mixed Indicator: Dissolve 0.066 g of methyl red and 0.099 g of bromocresol green in 100 ml of ethyl alcohol. Add 20 ml of this mixed indicator to each litre of 2% boric acid solution.

• 0.1M Potassium Hydrogen Phthalate: Dissolve 20.422 g of the salt in water and dilute to one litre. This is a primary standard and does not require standardization.

• 0.02M H2SO4: Prepare approximately 0.1M H2SO4 by adding 5.6 ml of conc. H2SO4 to about 1 litre of distilled water. From this, prepare 0.02M H2SO4 by diluting a suitable volume (20 ml made to 100 ml) with distilled water. Standardize it against 0.1M NaOH solution.

• 0.1M NaOH. Dissolve 4g NaOH in 100 ml distilled water. Standardize against potassium hydrogen phthalate.

Procedure 1.

2.

3.

4.

5.

6.

Weigh 20 g of soil sample in a 800 ml Kjeldahl flask.

Moisten the soil with about 10 ml of distilled water, wash down the soil, if any, adhering to the neck of the flask.

Add 100 ml of 0.32% of KMnO4 solution.

Add a few glass beads or broken pieces of glass rod.

Add 2-3 ml of paraffin liquid, avoiding contact with upper part of the neck of the flask.

Measure 20 ml of 2% boric acid containing mixed indicator in a 250 ml conical flask and place it under the receiver tube. Dip the receiver tube in the boric acid.

7. Run tap water through the condenser.

8. Add 100 ml of 2.5% NaOH solution and immediately attach to the rubber stopper fitted in the alkali trap.

9. Switch the heaters on and continue distillation until about 100 ml of distillate is collected.

10. First remove the conical flask containing distillate and then switch of the heater to avoid back suction.

11. Titrate the distillate against 0.02M H2SO4 taken in burette until pink colour starts appearing.

12. Run a blank without soil.

13. Carefully remove the Kjeldahl flask after cooling and drain the contents in the sink.

Calculation Volume of acid used to neutralize ammonia in the sample = A – B ml N content in the test sample (kg/ha) = (A – B) x 31.36 Where, A = Volume of 0.02N H2SO4 used in titration against ammonia absorbed in boric acid.

B = Volume of 0.02N H2SO4 used in blank titration.

1 ml of 0.02N H2SO4= 0.28 mg N (1 000 ml of 1NH2SO4 = 14 g Nitrogen).

Caution • • •

Check all the joints of the Kjeldahl apparatus to prevent any leakage and loss of ammonia.

Hot Kjeldahl flasks should neither be washed immediately with cold water nor allowed to cool for long to avoid deposits to settle at the bottom which are difficult to remove.

In case frothing takes place and passes through to the boric acid, such samples should be discarded and fresh distillation done.

18

• •

Opening ammonia bottles in the laboratory should be strictly prohibited while distillation is on. The titration should be carried out in ammonia free atmosphere.

In case the titration is not to be carried out immediately, the distillate should be stored in ammonia free cupboards after tightly stoppering the flasks

5. Available Phosphorus Two methods are most commonly used for determination of available phosphorus in soils: Bray‟s Method No.1 for acidic soils and Olsen‟s Method for neutral and alkaline soils. In these methods, specific coloured compounds are formed with the addition of appropriate reagents in the solution, the intensity of which is proportionate to the concentration of the element being estimated. The colour intensity is measured spectrophotometrically. In spectrophotometric analysis, light of definite wavelength (not exceeding say 0.1 to 1.0 nm in band width) extending to the ultraviolet region of the spectrum constitutes the light source. The photoelectric cells in spectrophotometer measure the light transmitted by the solution.

Bray’s method No. 1 (Bray and Kurtz, 1945) for acid soils Apparatus • Spectrophotometer • Pipette - 2 ml, 5 ml, 10 ml and 20 ml • Bearkers/flasks - 25 ml, 50 ml, 100 ml and 500 ml Reagents    

Bray Extractant No 1 (0.03M NH4F in 0.025M HCl): Dissolve 2.22 g of NH4F in 200 ml of distilled water, filter, and add to the filtrate 1.8 litres of water containing 4 ml of concentrated HCl, make up the volume to 2 litres with distilled water.

Molybdate reagent: Dissolve 1.50 g (NH4)2MoO4 in 300 ml distilled water. Add the solution to 350 ml of 10M HCl solution gradually with stirring. Dilute to 1 litre with distilled water.

Stannous chloride solution (Stock Solution): Dissolve 10 g SnCl2 2H2O in 25 ml of concentrated HCl. Add a piece of pure metallic tin and store the solution in a glass stoppered bottle.

Stannous chloride solution (Working Solution): Dilute 1 ml of the stock solution of stannous chloride to 66.0 ml with distilled water just before use. Prepare fresh dilute solution every working day.

Procedure Preparation of the Standard Curve: Dissolve 0.1916 g of pure dry KH2PO4 in 1 litre of distilled water. This solution contains 0.10 mg P2O5/ml. Preserve this as a stock standard solution of phosphate. Take 10 ml of this solution and dilute it to 1 litre with distilled water. This solution contains 1 µg P2O5/ml (0.001 mg P2O5/ml). Take 1, 2, 4, 6 and 10 ml of this solution in separate 25 ml flasks, which corresponds to 0.04, 0.08, 0.16, 0.24 and 0.40 ppm of phosphorus, respectively. Add to each, 5 ml of the extractant solution, 5 ml of the molybdate reagent and dilute with distilled water to about 20 ml. Add 1 ml dilute SnCl2 solution, shake again and dilute to the 25 ml mark. After 10 minutes, read the blue colour of the solution on the spectrophotometer at 660 nm wavelength. Final reading may be noted in ppm.

Extraction: Add 50 ml of the Bray‟s extractant No. 1 to the 100 ml conical flask containing 5 g soil sample. Shake for 5 minutes and filter.

Development of colour: 19

Take 5 ml of the filtered soil extract with a bulb pipette in a 25 ml measuring flask; deliver 5 ml of the molybdate reagent with an automatic pipette, dilute to about 20 ml with distilled water, shake and add 1 ml of the dilute SnCl2 solution with a bulb pipette. Fill to the 25 ml mark and shake thoroughly. Read the blue colour after 10 minutes on the spectrophotometer at 660 nm wavelength after setting the instrument to zero with the blank prepared similarly but without the soil.

Calculation P (kg/ha) = A*112 (Factor) Where, Factor= (A*25*50/5*5)*2.24= 112 A Weight of the soil taken = 5 g Volume of the extract = 50 ml Volume of the extract taken for estimation = 5 ml Volume made for estimation (dilution = 5 times) = 25 ml Amount of P observed in the sample on the standard curve = A (µg/ml).

Wt. of 1 ha of soil upto a plough depth is taken as 2.24 million kg.

Olsen’s method (Olsen, et al, 1954) for alkaline soils Apparatus Same as for Bray‟s Method No. 1.

Reagents • • • •

Bicarbonate extractant: Dissolve 42 g Sodium bicarbonate in 1 litre of distilled water and adjust the pH to 8.5 by addition of dilute NaOH or HCl. Filter, if necessary.

Activated carbon – Darco G 60.

Molybdate reagent: Same as for the Bray‟s Method No. 1.

Stannous chloride solution: Same as in Bray‟s Method No. 1.

Procedure 1. Preparation of the standard curve: Procedure is the same as in Bray‟s Method No. 1.

2. Extraction: Add 50 ml of the bicarbonate extractant to 100 ml conical flask, containing 2.5 g soil sample. Add 1 g activated carbon. Shake for 30 minutes on the mechanical shaker and filter.

3. Development of Colour: Procedure same as described under the Bray‟s Method No. 1.

Calculation Same as described under the Bray‟s Method No. 1.

Caution: In spite of all precautions, intensity of blue colour changes slightly with every batch of molybdate reagent. It is imperative to check standard curve every day by using 2 or 3 dilutions of the standard phosphate solution. If the standard curve does not tally, draw a new standard curve with fresh molybdate reagent.

Ascorbic acid method of phosphorus estimation 20

The major drawback with the blue colour method (Dickman and Bray 1940) is that the colour starts fading soon, and hence the intensity has to be measured quickly. A method based on the use of ascorbic acid (watanabe and Olsen, 1965) as described below provides a more stable blue colour and is, therefore, preferred over the former.

Reagents 1. Molybdate-tatrate solution: Dissolve 12 g of ammonium molybdate in about 250 mL of distilled water to get solution „A‟.

Prepare solution „B‟ by dissolving 0.291 g of antimony potassium tartrate in 100 mL of distilled water. Preprare one litre of 5N H2SO4 and add solutions „A‟ and “B‟ to it. Mix thoroughly and make the volume to 2 litres with distilled water.

2. Ascorbic acid solution: Dissolve 1.056 g of ascorbic acid in 200 mL of the molybdate-tartrate solution and mix wel. Prepare it fresh as and when required.

3. P-nitrophenol indicator: Dissolve 0.5 g of p-nitrophenol in 100 mL of distilled water.

4. 5N H2SO4: Carefully dilute 140 mL of conc. H2SO4 to 1 L with distilled water (H2SO4 to be slowly added to water) to get approx. 5 N H2SO4.

Procedure 1. Pipette 5 mL of the Olsen‟s reagent into a 25 ml volumetric flask and held 2-3 drops of p-nitrophenol indicator. It develops yellow colour.

2. Add known quality of 5N H2SO4 drop by drop to acidify the Olsen‟s reagent to Ph 5.0 at which the yellow colour will disappear. Note the volume of 5N H2SO4 used.

3. Transfer of 5 mL aliquot of the Olsen‟s extract of the soil to a 25 mL volumetric flask and add the required quantity of 5N H2SO4 to bring it to pH 5.0.

4. Dilute o 20 mL with distilled water.

5. Add 4 mL of the ascorbic acid solution make the volume to 25mL and shake well.

6. Wait for 10 minute and then measure the colour intensity at 730-840 nm.

7. Run a blank with the extracting solution (without soil) 8. Prepare standard curve as described for Dickman and Bray‟s method.

Note 1. All glassware to be used for P determination should be cleaned with chromic acid, followed by thorough washing with water to minimize contamination.

2. In case of the delay in measuring colour intensity, covers the samples, store properly and add SnCl2 just before measurements.

3. Colour intensity measurement should be carried out exactly 10 minute after developing blue colour.

6. Available Potassium Flame photometeric method (Toth and Prince, 1949) Potassium present in the soil is extracted with neutral ammonium acetate of 1 molarity. This is considered as plant available K in the soils. It is estimated with the help of flame photometer.

Apparatus • Multiple Dispenser or automatic pipette – 25 ml • Flasks and beakers - 100 ml • Flame Photometer 21

Reagents •

Molar neutral ammonium acetate solution: Dissolve 77 g of ammonium acetate (NH4C2H3O2) in 1 litre of water. Check the pH with bromothymol blue or with a pH meter. If not neutral, add either ammonium hydroxide or acetic acid as per the need to neutralize it to pH 7.0.

• Standard potassium solution: Dissolve 1.908 g pure KCl in 1 litre of distilled water. This solution contains 1 mg K/ml . Take 100 ml of this solution and dilute to 1 litre with ammonium acetate solution. This gives 0.1 mg K/ml as stock solution.

• Working potassium standard solutions: Take 0, 5, 10, 15 and 20 ml of the stock solution separately and dilute each to 100 ml with the M ammonium acetate solution. These solutions contain 0, 5, 10, 15 and 20 µg K/ml, respectively.

Procedure 1. Preparation of the Standard Curve: Set up the flame photometer by atomizing 0 and 20 µg K/ml solutions alternatively to 0 and 100 reading. Atomize intermediate working standard solutions and record the readings. Plot these readings against the respective potassium contents and connect the points with a straight line to obtain a standard curve.

2. Extraction: Add 25 ml of the ammonium acetate extractant to conical flask fixed in a wooden rack containing 5 g soil sample.

Shake for 5 minutes and filter.

3. Determine potash in the filtrate with the flame photometer.

Calculation K (kg/ha) = A x (25/5) x (2.24) = 11.2 A Where, A = content of K (µg/ml) in the sample, as read from the standard curve: Weight of 1 ha of soil depth of furrow slice is approx. 2.24 million kg.

7. Available Sulphur Available sulphur in mineral soils occurs mainly as adsorbed SO4 ions. Phosphate ions (as monacalcium phosphate) are generally preferred for replacement of the adsorbed SO4 ions. The extraction is also carried out using CaCl2 solution. The former is considered to be better for more efficient replacement of SO4 ions. However, use of Ca salt has a distinct advantage over those of Na or K as Ca prevents deflocculation in heavy textured soils and leads to easy filtration. SO4 in the extract can be estimated turbidimetrically using a spectrophotometer. A major problem arises when the amount of extracted sulphur is too low to be measured. To overcome this 101 problem, seed solution of known S concentration is added to the extract to raise the concentration to easily detectable level.

Barium sulphate precipitation method is described here.

Apparatus • Spectrophotometer • Mechanical shaker • Volumetric flask Reagents • • • •

Mono-calcium phosphate extracting solution (500 mg P/litre): Dissolve 2.035 g of Ca(H2PO4)2.H2O in 1 litre of water.

Gum acacia-acetic acid solution: Dissolve 5g of chemically pure gum acacia powder in 500 ml of hot water and filter in hot condition through Whatman No.42 filter paper. Cool and dilute to one litre with dilute acetic acid.

Barium chloride: Pass AR grade BaCl2 salt through 1 mm sieve and store for use.

Standard stock solution (2000 mg S/litre): Dissolve 10.89g of oven-dried AR grade potassium sulphate in 1 litre water.

22

• • • •

Standard working solution (10 mg S/litre): Measure exactly 2.5 ml of the stock solution and dilute to 500 ml.

Barium sulphate seed suspension: Dissolve 18 g of AR grade BaCl2 in 44 ml of hot water and add 0.5 ml of the standard stock solution. Heat the content to boiling and then cool quickly. Add 4 ml of gum acacia-acetic acid solution to it. Prepare a fresh seed suspension for estimation everyday.

Dilute nitric acid (approx 25%): Dilute 250 ml of AR grade conc. HNO3 to one litre.

Acetic-phosphoric acid: Mix 900 ml of AR grade glacial acetic acid with 300 ml of H3PO4 (AR grade).

Procedure 1. Weigh 20 g of soil sample in a 250 ml conical flask. Add 100 ml of the monocalcium phosphate extracting solution (500 mg P/litre) and shake for one hour. Filter through Whatman No.42 filter paper.

2. Take 10 ml of the clear filtrate into a 25 ml volumetric flask.

3. Add 2.5 ml of 25% HNO3 and 2 ml of acetic-phosphoric acid. Dilute to about 22 ml, stopper the flask and shake well, if required.

4. Shake the BaSO4 seed suspension and then add 0.5 ml of it, and 0.2 g of BaCl2 crystals. Stopper the flask and invert three times and keep.

5. After 10 minutes, invert 10 times. Again after 5 minutes invert for 5 times.

6. Allow to stand for 15 minutes and then add 1 ml of gum acacia-acetic acid solution.

7. Make up the volume to 25 ml, invert 3 times and keep aside for 90 minutes.

8. Invert 10 times and measure the turbidity intensity at 440 nm (blue filter).

9. Run a blank side by side.

10. Preparation of standard curve: • Put 2.5, 5.0, 7.5, 10.0, 12.5, 15.0 ml of the working standard solution (10 mg S/litre) into a series of 25 ml volumetric flasks to obtain 25, 50, 75, 100, 125 and 150 µg S. • Proceed to develop turbidity as described above for sample aliquots.

• Read the turbidity intensity and prepare the curve by plotting readings against sulphur concentrations (in µg in the final volume of 25 ml).

Calculation Available Sulphur (SO4-S) in soil (mg/kg) = 12.5* A Factor (12.5)= (100/20)*(25/10)= 12.5 Where A = content of K (µg/ml) in the sample, as read from the standard curve: Where, W stands for the quantity of sulphur in mg as obtained on X-axis against an absorbance reading (Y-axis) on standard curve 20 is the weight of the soil sample in g 100 is the volume of the extractant in ml 10 is the volume of extractant in ml in which turbidity is developed.

8A. Calcium by Versenate (EDTA) method Apparatus • Shaker • Porcelain dish • Beakers • Volumetric/conical flask.

Reagents 23

• • • • • •

Ammonium chloride – ammonium hydroxide buffer solution: Dissolve 67.5 g ammonium chloride in 570 ml of conc. ammonium hydroxide and make to 1 litre.

Standard 0.01N calcium solution: Take accurately 0.5 g of pure calcium carbonate and dissolve it in 10 ml of 3N HCl. Boil to expel CO2 and then make the volume to 1 litre with distilled water.

EDTA solution (0.01N): Take 2.0 g of versenate, dissolve in distilled water and make the volume to 1 litre. Titrate it with 0.01N calcium solution and make necessary dilution so that its normality is exactly equal to 0.01N. Muroxide indicator powder: Take 0.2 g of muroxide (also known as ammonium purpurate) and mix it with 40 g of powdered potassium sulphate. This indicator should not be stored in the form of solution, otherwise it gets oxidized.

Sodium diethyl dithiocarbamate crystals: It is used to remove the interference of other metal ions.

Sodium hydroxide 4N: Prepare 16% soda solution by dissolving 160 g of pure sodium hydroxide in water and make the volume to 1 litre. This will give pH 12.

Procedure 1. Take 5 g air dried soil sample in 150 ml conical flask and add 25 ml of neutral normal ammonium acetate. Shake on mechanical shaker for 5 minutes and filter through Whatman filter paper No.1.

2. Take a suitable aliquot (5 or 10 ml) and add 2-3 crystals of carbamate and 5 ml of 16% NaOH solution.

3. Add 40-50 mg of the indicator powder. Titrate it with 0.01N EDTA solution till the colour gradually changes from orange red to reddish violet (purple). It is advised to add a drop of EDTA solution at an interval of 5 to 10 seconds, as the change of colour is not instantaneous.

4. The end point must be compared with a blank reading. If the solution is over titrated, it should be back titrated with standard calcium solution and exact volume used is thus found.

5. Note the volume of EDTA used for titration.

Calculation If N1 is normality of Ca2+ and V1 is volume of aliquot taken and N2V2 are the normality and volume of EDTA used, respectively, then, N1V1 = N2V2 Or N1 = (N2V2)/V1= (Normalityof EDTA x Vol of. EDTA)/ ml of aliquot takenHere N1 (Normality) = equivalent of Ca2+ present in one litre of aliquot. Hence, Ca (meq/litre) = (Noramlity of EDTA x Vol.of EDTA x1000)/ ml of aliquot taken When expressed on soil weight basis, Ca 2+ me/100 g soil = (100/Wt. of soil) x (extract volume/1000) x Ca as meq/litre

8B. Calcium plus Magnesium by Versenate (EDTA) method Magnesium in solution can be titrated with 0.01N EDTA using Eriochrome black T dye as indicator at pH 10 in the presence of ammonium chloride and ammonium hydroxide buffer. At the end point, colour changes from wine red to blue or green. When calcium is also present in the solution this titration will estimate both calcium and magnesium. Beyond pH 10 magnesium is not bound strongly to Erichrome black T indicator to give a distinct end point.

Apparatus • Shaker • Porcelain dish • Beaker • Volumetric/conical flask Reagents •

EDTA or Versenate solution (0.01N): Same as in calcium determination.

24

• •

Ammonium chloride-ammonium hydroxide buffer solution: Same as in calcium determination.

Eriochrome black T indicator: Take 100 ml of ethanol and dissolve 4.5 g of hydroxyl amine hydrochloride in it. Add 0.5 g of the indicator and prepare solution. Hydroxylamine hydrochloride removes the interference of manganese by keeping it in lower valency state (Mn2+). Or mix thoroughly 0.5 g of the indicator with 50 g of ammonium chloride.

• Sodium cyanide solution (2%) or sodium diethyl dithiocarbamate crystals. This is used to remove the interference of copper, cobalt and nickel.

Procedure 1. Take 5 g air dried soil in 150 ml flask, add 25 ml of neutral normal ammonium acetate solution and shake on a mechanical shaker for 5 minutes and filter through Whatman No.1 filter paper.

2. Pipette out 5 ml of aliquot containing not more than 0.1 meq of Ca plus Mg. If the solution has a higher concentration, it should be diluted.

3. Add 2 to 5 crystals of carbamate and 5 ml of ammonium chloride-ammonium hydroxide buffer solution. Add 3-4 drops of Eriochrome black T indicator.

4. Titrate this solution with 0.01N versenate till the colour changes to bright blue or green and no tinge of wine red colour remains.

Calculation If N1 and V1 are normality (concentration of Ca +++Mg ++) and volume of aliquot taken and N2V2 are the normality and volume of EDTA used respectively, then, N1V1 = N2V2 Or N1 =(N2V2)/V1 = (Normality of EDTAxVol. of EDTA)/( ml of aliquot taken) Here N1 (Normality) = equivalents of Ca ++ plus Mg ++ present in one litre of aliquot.

Meliequivalent (me) of Mg++ = me (Ca++ + Mg++) – me of Ca++ When expressed on soil weight basis.

Ca++ + Mg++ me/100 g soil = (100/Wt. of soil) x (Extract volume/1000) x Ca++ + Mg++ me/Litre

9. Micronutrients For estimation of micronutrients also, it is the plant available form which is critical and not the total content. The major objective of soil test for micronutrients, like macronutrients, is to determine whether a soil can supply adequate micronutrients for optimum crop production or whether nutrient deficiencies are expected in crops grown on such soils. Most commonly studied micronutrients are Zn, Cu, Fe, Mn, B and Mo and the same have been dealt with here.

Micronutrients are present in different forms in the soil. Among the most deficient ones is Zn, which is present as divalent cation Zn2+. Maize, citrus, legumes, cotton and rice are especially sensitive to zinc deficiency. Iron is present mostly in sparingly soluble ferric oxide form, which occurs as coatings of aggregate or as separate constituent of the clay fraction. Soil redox potential and pH affect the availability of iron. The form of iron that is predominantly taken up by plants is the Fe2+. Uptake of Fe is inhibited by phosphate levels due to the formation of insoluble iron phosphate. ml of aliquot taken. Manganese, chemically behaves in the soils the same way as Fe. Soil Mn originates primarily from the decomposition of ferromagnesian rocks. It is taken up by the plants as Mn2+ ions, although it exists in many oxidation states. Manganese and phosphate are mutually antagonistic. Copper like zinc exists in soils mainly as divalent ions Cu2+. It is usually adsorbed by the clay minerals or associated with organic matter, although they have little or no effect on its availability to crops. High phosphate fertilization can induce Cu deficiency. Molybdenum mostly occurs as MoO3, MoO5 and MoO2. These oxides are slowly transformed to soluble molybdates (MoO4) which is the form taken up by plants.

Boron deficiency occurs mostly in the light textured acid soils when they are leached heavily through irrigation or heavy rainfall.

25

Different extractants have been developed for assessing plant available nutrient (element) content in the soils. The elements so extracted can be estimated quantitatively through chemical methods or instrumental techniques.

Available Zinc, Copper, Iron and Manganese Ethylene damine-tera acitic acid (EDTA) + Ammonium Acetate is commonly used for extraction of many elements. DTPA (Diethylenetriaminepentaacetic acid) is yet another common (universal) extractant and is widely used for simultaneous extraction of elements, like Zn, Cu, Fe and Mn. This extractant was advanced by Lindsay and Norvell (1978). Therefore, DTPA being one such extractant has been described here. The estimation of elements in the extract is done with the help of Atomic Absorption Spectrophotometer (AAS).

Principle of extraction DTPA is an important and widely used chelating agent, which combines with free metal ions in the solution to form soluble complexes of elements. To avoid excessive dissolution of CaCO3, which may release occluded micronutrients that are not available to crops in calcarious soils and may give erroneous results, the extractant is buffered in slightly alkaline pH. Triethanolamine (TEA) is used as buffer because it burns cleanly during atomization of extractant solution while estimating on AAS. The DTPA has a capacity to complex each of the micronutrient cations as 10 times of its atomic weight. The capacity ranges from 550 to 650 mg/kg depending upon the micronutrient cations.

Extracting solution (DTPA) DTPA 0.005M, 0.01M CaCl2.2H2O and 0.1M TEA extractant: Add 1.967 g DTPA and 13.3 ml TEA in 400 ml distilled water in a 500 ml flask. Take 1.47 g CaCl2.2H2O in a separate 1000 ml flask. Add 500 ml distilled water and shake to dissolve. Add DTPA+TEA mixture in CaCl2 solution and make the volume to 1 litre. pH should be adjusted to 7.3 by using 1M HCl before making the volume.

Principle of Estimation The extracted elements can be estimated by various methods, which include volumetric analysis, spectrometry and atomic absorption spectroscopy. Volumetric methods such as EDTA and KMnO4 titrations are used for estimation of zinc and Mn, and iron, respectively.

Copper can be estimated by titration with Na2S2O3. Spectrometric methods are deployed in estimation of specific colour developed due to the presence of an element, which forms coloured compound in the presence of specific chemicals under definite set of conditions.

The colour intensity has to be linear with the concentration of the element in question. The interference due to any other element has to be eliminated. Such methods are dithiozone method for estimation of zinc, orthophenonthroline method for iron, potassium periodate method for manganese, carbamate method for copper. The chemical methods are generally cumbersome and time taking.

Hence the most commonly employed method is atomic absorption spectrometry. Here, the interference by other elements is almost nil or negligible because the estimation is carried out for an element at a specific emission spectra. In fact in AAS, traces of one element can be accurately determined in the presence of a high concentration of other elements.

Preparation of standard solutions Readymade standard solutions 1000 µg/ml or 1 mg/ml (1000 ppm) of dependable accuracy are supplied with the AAS and are also available with the suppliers of chemical reagents. If the standard solutions are to be prepared in the laboratory, either metal element foils of 100% purity or the standard chemical salts can be used. The quantities of chemical required to make 1 litre standard solution of 100 µg/ml for different elements are given below.

Element

Conc. of stock Solution (µg/ml)

Salt to be used

Quantity of salt required/litre 26

Zn Cu Fe

100 100 100

Zinc Sulphate (ZnSO4.7H2O) Copper Sulphate (CuSO4.5H2O) Ferrous Sulphate (FeSO4.7H2O) or

0.4398 0.3928 0.4964

Mn

100

Ferrous Ammonium Sulphate Manganese Sulphate (MnSO4.H2O)

0.7028 0.3075

In case of Zn, Cu and Fe, 1000 µg/ml (1000 ppm) standard solution are preferably prepared by dissolving 1.0 g pure metal wire and volume made to 1 litre as per the method described under each element. It is diluted to obtain the required concentration. In case of Mn, Mn SO4.H2O is preferred.

Preparation of standard curves i. Zinc Reagents •

• •

Standard Zinc Solution: Weigh 1.0 g of pure zinc metal in a beaker. Add 20 ml HCl (1:1). Keep for few hours allowing the metal to dissolve completely. Transfer the solution to 1 litre volumetric flask. Make up the volume with glass-distilled water. This is 1000 µg/ml zinc solution. For preparation of standard curve, refer 1000 µg/ml solution as solution A. Dilute 1 ml of standard A to 100 ml to get 10 µg/ml solution to be designated as standard B. Glass-distilled or demineralized acidified water of pH 2.5 + 0.5: Dilute 1 ml of 10% sulphuric acid to one litre with glassdistilled or mineralized water and adjust the pH to 2.5 with a pH meter using 10% H2SO4 or NaOH. This solution is called acidified water.

Working Zn standard solutions: Pipette 1, 2, 4, 6, 8 and 10 ml of standard B solution in 50 ml numbered volumetric flask and make the volume with DTPA solution to obtain 0.2, 0.4, 0.8, 1.2, 1.6 and 2.0 µg/ml zinc. Stopper the flasks and shake them well. Fresh standards should be prepared every time when a fresh lot of acidified water is prepared.

Procedure 1. Flaming the solutions: Atomise the standards on atomic absorption spectrophotometer at a wave-length of 213.8 nm (Zn line of the instrument).

2. Prepare a standard curve of known concentrations of zinc solution by plotting the absorbance values on Y–axis against their respective zinc concentration on X–axis.

Precautions • Weighing must be done on an electronic balance.

• All the glass apparatus to be used should be washed first with dilute hydrochloric acid (1:4) and then with distilled water.

• The pipette should be rinsed with the same solution to be measured.

• The outer surface of the pipette should be wiped with filter paper after use.

• After using the pipette, place them on a clean dry filter paper in order to prevent contamination. ii. Copper Reagents •

Standard copper solution: Weigh 1 g of pure copper wire on a clean watch glass and transfer it to one litre flask. Add 30 ml of HNO3 (1:1) and make up the mark. Stopper the flask and shake the solution well. This is 1 000 µg/ml Cu solution and should be stored in a clean bottle for further use. Dilute 1 ml of 1000 µg/ml solution of copper to 100 ml to get 10 µg/ml standard copper solution.

27

• •

Glass-distilled or demineralized acidified water of pH 2.5 + 0.5: Same as that done for Zn. Working Cu standard solutions: Pipette 2, 3, 4, 5, 6 and 7 ml of 10 µg/ml standard Cu solution in 50 ml numbered volumetric flasks and make the volume with DTPA solution to get 0.4, 0.6, 0.8, 1.0, 1.2 and 1.4 µg/ml copper. Stopper the flasks and shake them well. Prepare fresh standards every fortnight.

Procedure 1. Flame the standards on an atomic absorption spectrophotometer at a wavelength of 324.8 nm (Cu line of the instrument). 2.

Prepare the standard curve with the known concentration of copper on X-axis by plotting against absorbance value on Y-axis. iii. Iron Reagents •

• •

Standard iron solution: Weigh accurately 1 g pure iron wire and put it in a beaker and add approximately 30 ml of 6M HCl and boil. Transfer it to one litre volumetric flask through the funnel giving several washings to the beaker and funnel with glassdistilled water. Make the volume up to the mark. Stopper the flask and shake the solution well. This is 1 000 µg/ml iron solution.

Glass-distilled or demineralized acidified water of pH 2.5 ± 0.5: Same as that done for Zn. Working Fe standard solutions: Pipette 10 ml of iron stock solution in 100 ml volumetric flask and dilute to volume with DTPA solution. This is 100 µg/ml iron solution. Take 2, 4, 8, 12 and 16 ml of 100 µg/ml solution and dilute each to 100 ml to obtain 2, 3, 8, 12 and 16 µg/ml of Fe solution.

Procedure 1. Flame the standards on an atomic absorption spectrophotometer at a wavelength of 248.3 nm (Fe line of the instrument). 2.

Prepare the standard curve with the known concentration of copper on X-axis by plotting against absorbance value on Y-axis. iv Manganese Reagents •

Standard Mn solution: Weigh 3.0751 g of AR grade manganese sulphate (MnSO4 H2O) on a clean watch glass and transfer it to one litre flask through the funnel giving several washings to watch glass and funnel with acidified water and make the volume up to the mark. This solution will be 1 000 µg/ml Mn. A secondary dilution of 5 ml to 100 ml with acidified water gives a 50 µg/ml solution.

• Glass-distilled or de-mineralized acidified water of pH 2.5 + 0.2: Same as that for Zn. • Working Mn standard solutions: Standard curve is prepared by taking lower concentrations of Mn in the range of 0-10 µg/ml. Take 1, 2, 4, 6 and 8 ml of 50 µg/ml solution and make up the volume with DTPA solution to 50 ml to obtain 1, 2, 4, 6 and 8 µg/ml working standards.

Procedure 1. Flame the standards on an atomic absorption spectrophotometer at a wavelength of 279.5 nm (Mn line of the instrument). 2.

Prepare the standard curve with the known concentration of Mn on X-axis by plotting against absorbance value on Y-axis.

Procedure for extraction by DTPA Once standard curves have been prepared, proceed for extraction by DTPA. 1. Take 10 g of soil sample in 100 ml narrow-mouth polypropylene bottle.

28

2.

3.

4.

5.

Add 20 ml of DTPA extracting solution.

Stopper the bottle and shake for 2 hours at room temperature (250 C).

Filter the content using filter paper No.1 or 42 and collect the filtrate in polypropylene bottles Prepare a blank following all steps except taking a soil sample.

Note: The extract so obtained is used for estimation of different micronutrients. For extraction of more accurate quantity of an element which has a higher degree of correlation with plant availability, there are element specific extractants. An extractant standardized/recommended for a given situation in a country may be used. The estimation procedure on AAS, however, remains unchanged.

Estimation on AAS 1.

2.

3.

4.

5.

Select an element specific hollow cathode lamp and mount it on AAS. Start the flame.

Set the instrument at zero by using blank solution.

Aspirate the standard solutions of different concentrations one by one and record the readings.

Prepare standard curve plotting the concentration of the element concerned and the corresponding absorbance in different standard samples (as described before).

6. When the operation is performed accurately, a straight line relationship is obtained between the concentration of the element and the absorbance on AAS with a correlation coefficient which may be nearly as high as 1.0.

7. Aspirate the soil extractant obtained for estimation of nutrient element in the given soil sample and observe the readings.

8. Find out the content of the element in the soil extract by observing its concentration on the standard curve against its absorbance.

Calculation Content of micronutrient in the sample (mg/kg) = C µg/ml x 2 (dilution factor).

Where, Dilution factor = 2.0 (Soil sample taken = 10.0 g and DTPA used = 20 ml) Absorbance reading on AAS of the soil extract being estimated for a particular element =X Concentration of micronutrient as read from the standard curve for the given absorbance (X) = C µg/ml.

10. Determination of Available Boron The most commonly used method for available B is hot water extraction of soil as developed by Berger and Truog (1939). Water soluble boron is the available form of boron. It is extracted from the soil by water suspension. In the extract, boron can be analysed by colorimetric methods using reagents such as Carmine, azomethine – H and most recently by inductively coupled plasma (ICP) and atomic emission spectrometry (Haubold et al., 1988; Jeffrey and McCallum, 1988). Colourimetric method is however, preferable due to the fact that boron being a non-metal, use of AAS for its estimation pose some limitations. The extraction method described here is the simple modification (Gupta, 1967) of the one developed by Berger and Truoug (1939) in which boiling soil with water is employed.

Extraction Procedure 1. Weight 25 g of soil in a quartz flask or beaker.

2. Add about 50 ml of double distilled water and about 0.5 g of activated charcoal.

3. The mixture is boiled for about 5 minutes and filtered through Whatman Filter Paper No.42.

29

Estimation by colorimetric method The extracted B in the filtered extract is determined by the azomethine-H colorimetric method.

Apparatus • Analytical balance • Flask or beaker • Volumetric flask • Funnels • Whatman No.42 filter paper • Spectrophotometer Reagents • • • • •

Azomethine-H: Dissolve 0.45 g azomethine-H and 1.0 g L-ascorbic acid in about 100 ml deionized or double-distilled water. If solution is not clear, it should be heated gently in a water bath or under a hot water tap at about 300 C till it dissolves. Every week a fresh solution should be prepared and kept in a refrigerator.

Buffer solution: Dissolve 250 g ammonium acetate in 500 ml deionized or double-distilled water and adjust the pH to about 5.5 by slowly adding approximately 100 ml glacial acetic acid, with constant stirring.

EDTA solution (0.025 M): Dissolve 9.3 g EDTA in deionized or double-distilled water and make the volume upto 1 litre.

Standard stock solution: Dissolve 0.8819g Na2B4O710H2O AR grade in a small volume of deionized water and make volume to 1 000 ml to obtain a stock solution of 100 µg B/ml.

Working standard solution: Take 5 ml of stock solution in a 100 ml volumetric flask and dilute it to the mark. This solution contains 5 µg B/ml.

Procedure 1. Take 5 ml of the clear filtered extract in a 25 ml volumetric flask and add 2 ml buffer solution, 2 ml EDTA solution and 2 ml azomethine-H solution.

2. Mix the contents thoroughly after the addition of each reagent.

3. Let the solution stand for 1 hour to allow colour development. Then, the volume is made to the mark.

4. Intensity of colour is measured at 420 nm.

5. The colour thus developed has been found to be stable upto 3-4 hours.

6. Preparation of standard curve: Take 0, 0.25, 0.50, 1.0, 2.0 and 4.0 ml of 5 µg B/ml solution (working standard) to a series of 25 ml volumetric flasks. Add 2 ml each of buffer reagent, EDTA solution and azomethine-H solution. Mix the contents after each addition and allow to stand at room temperature for 30 minutes. Make the volume to 25 ml with deionized or double-distilled water and measure absorbance at 420 nm. This will give reading for standard solution having B concentration 0, 0.05, 0.10, 0.20, 0.40 and 0.80 µg B/ml.

Calculation Content of B in the soil (µg/g or mg/kg) = C x Dilution factor (10) Where, C (µg/ml) = Concentration of B as read from the standard curve against the absorbance reading of the soil solution on the spectrophotometer.

Dilution factor = 10 which is calculated as follows: • Weight of the soil taken = 25 g • Volume of extractant (water) added = 50 ml • First dilution = 2 times • Volume of the filtrate taken = 5 ml • Final volume of filtrate after colour development = 25 ml 30

• Second dilution = 5 times • Total dilution = 2 x 5 = 10 times Note: 1. The use of azomethine-H is an improvement over that of carmine, quinalizarin and curcumin, since the procedure involving this chemical does not require the use of concentrated acid.

2. The amount of charcoal added may vary with the organic matter content of the soil and should be just sufficient to produce a colourless extract after 5 min. of boiling on a hot plate. Excess amounts of charcoal can result in loss of extractable B from soils.

11. Determination of Available Molybdenum Molybdenum (Mo) is a rare element in soils, and is present only in very small amounts in igneous and sedimentary rocks. The major inorganic source of Mo is molybdenite (MoS2). The total Mo content in soils is perhaps the lowest of all the micronutrient elements, and is reported to range between 0.2 µg/g and 10 µg/g. In the soil solution Mo exists mainly as HMoO4 ion under acidic condition, and as MoO42- ion under neutral to alkaline conditions. Because of the anionic nature of Mo, its anions will not be attracted much by the negatively charged colloids, and therefore, tend to be leached from the soils in humid region. Molybdenum can be toxic due to greater solubility in alkaline soils of the arid and semi-arid regions, and deficient in acid soils of the humid regions. In plants a deficiency of Mo is common at levels of 0.1 µg/g soil or less. Molybdenum toxicity (molybdenosis) is common when cattle graze forage plants with 10-20 µg Mo/g.

In case of Molybdenum, ammonium acetate and/or ammonium oxalate extraction is usually carried out. Estimations can be done both

by AAS and colourimetric methods with preference for the latter due to the formation of oxide in the flame in case of estimation by AAS. Therefore, chemical method has also been described. Ammonium oxalate is considered as a better extractant. However, for estimation on AAS ammonium acetate is preferred as the oxalates pose a limitation on AAS unless removed by digesting with diacid as is described in case of colorimetric estimation.

Estimation by colorimetric method Apparatus • Spectrophotometer • Hot plate • Refrigerator • Water bath Reagents • • • •

• • • • •

50% potassium iodide solution: Dissolve 50 g in 100 ml of double-distilled water (DDW).

50% ascorbic acid solution: Dissolve 50 g in 100 ml of DDW. 10% sodium hydroxide solution: Dissolve 10 g of NaOH in 100 ml of DDW. 10% thiourea solution: Dissolve 10 g in 100 ml of DDW and filter. Prepare fresh solution on the same day of use. • Toluene-3, 4dithiol solution (commonly called dithiol): Weigh 1.0 g of AR grade melted dithiol (510 C) in a 250 ml glass beaker. Add 100 ml of the 10% NaOH solution and warm the content upto 510 C with frequent stirring for 15 minutes. Add 1.8 ml of thioglycolic acid and store in a refrigerator.

10% tartaric acid: Dissolve 10 g in 100 ml of DDW. Iso-amyl acetate.

Ethyl alcohol.

Ferrous ammonium sulphate solution: Dissolve 63 g of the salt in about 500 ml of DDW and then make the volume to one litre.

Nitric-perchloric acid mixture (4:1) 31

• • •

Extracting reagent: Dissolve 24.9 g of AR grade ammonium oxalate and 12.6 g oxalic acid in water and make the volume to one litre.

Standard stock solution (100 µg/ml Mo): Dissolve 0.150 g of AR grade MoO3 in 100 ml of 0.1M NaOH, make slightly acidic with dilute HCl and make the volume to 1 litre.

Working standard solution (1 µg/ml Mo): Dilute 10 ml of the stock solution to 1 litre.

Procedure 1. Weigh 25 g of air-dry soil sample in a 500 ml conical flask. Add 250 ml of the extracting solution (1:10 ratio) and shake for 10 hours.

2. Filter through Whatman No.50 filter paper. Collect 200 ml of the clear filtrate in a 250 ml glass beaker and evaporate to dryness on a water bath.

3. Heat the contents in the beaker at 5000 C in a furnace for 5 hours to destroy organic matter and oxalates. Keep overnight.

4. Digest the contents with 5 ml of HNO3-HClO4 mixture (4:1), then with 10 ml of 4M H2SO4 and H2O2, each time bringing to dryness.

5. Add 10 ml of 0.1M HCl and filter. Wash the filter paper,first with 10 ml of 0.1M HCl and then with 10 ml of DDW until the volume of the filtrate is 100 ml.

6. Run a blank side by side (without soil).

7. Take 50 ml of the filtrate in 250 ml separatory funnels and add 0.25 ml of ferrous ammonium sulphate solution and 20 ml of DDW and shake vigorously.

8. Add excess of potassium iodide (Kl) solution and clear the liberated iodine by adding ascorbic acid drop by drop while shaking vigorously.

9. Add one ml of tartaric acid and 2 ml of thiourea solution and shake vigorously.

10. Add 5 drops of dithiol solution and allow the mixture to stand for 30 minutes.

11. Add 10 ml of iso-amyl acetate and separate out the contents (green colour) in colorimeter tubes/cuvettes.

12. Read the colour intensity at 680 nm (red filter).

13. Preparation of standard curve: Measure 0, 2, 5, 10, 15 and 20 ml of the working standard Mo solution containing 1 mg/litre Mo in a series of 250 ml separatory funnels. Proceed for colour development as described above for sample aliquots. Read the colour intensity and prepare the standard curve by plotting Mo concentration against readings.

Calculation Available Mo in soil (mg/g) = A x (250/200) x 1/25 = A/20 Where, A = Mo concentration in µg / ml as obtained on X-axis against a sample reading.

12. Measuring Soil Depth This method follows Lafitte (1994). Measure the depth in two or three places in a field though one place may be adequate if the crop looks uniform and is on a level area. If the crop looks different in different areas of the field, you will need to measure soil depth in each area.

To measure soil depth you will need a shovel. A soil corer can be used instead, but a corer can miss hardpans if the user is not familiar with the soils in an area. In addition, a corer can often hit stones in rocky soils, making the user think that the soil is shallower than it actually is. Pick an area where the plants look uniform, and start digging with a spade, preferably when the soil is moist (near field capacity), as digging in dry soil can be difficult. Dig until you strike any barrier. This can be a plow pan, a hard pan, a rock layer, an impermeable clay layer, acid subsoil, a water table, or a salt band. If you are uncertain that what you find is a

32

barrier, examine the roots of the crop to see if they penetrate the area. Scraping along the face of the hole with a pocket knife will often help in detecting a compacted layer. Record this depth.

Use a soil corer below that depth, taking several cores from the bottom of the hole you dug in order to avoid possible errors caused by striking rocks. Break the cores at several places to see if roots are still present at different depths.

13. Determination of Soil Texture (Feel Method) Soil texture is qualitatively determined by rapid feel method which involves rubbing the moistened soil between thumb and forefingers. In this procedure proficiency is gained through practice, and making comparison with samples of known textural class determined by some quantitative methods. Each soil separate has a distinct texture that can be distinguished by touch. Sand feels gritty and one can see individual grains with the naked eye. Silt will feel smooth almost silky, but one can only see individual particles under a microscope. Clay is the smallest particle. It will feels sticky when wet and can easily be molded into long ribbons.

Take a sample of soil and remove the > 2 mm fraction by sieving or by hand. Moisten the soil with a little water. The soil should be sufficiently moist. Mix thoroughly on a glass or in porecelain dish to form a soft ball. Move the soil between your thumb and forefinger to determine if the soil feels gritty, sticky, or smooth. The feel to fingers, ease of forming ball, stickiness or grittiness, whether forming soil ribbons or merely crumbling on squeezing etc. are observed. The sample is assigned approximate textural class as follows: Very Coarse Texture Sand: Very gritty, does not form ball, does not stain finger.

Coarse Texture Loamy sand: Very gritty, forms ball but very easily broken, stains fingers slightly.

Sandy loam: Moderately gritty, forms fairly form ball, which is easily broken, definitely stains fingers.

Medium Texture Loam: Neither very gritty nor very smooth, forms firm ball but does not ribbon, stains finger appreciably.

Silt loam: Smooth or sticky, buttery feel, forms firm ball, stains and has a slight tendency to ribbon with flaky surface.

Fine Texture Clay loam: Moderately sticky, slightly gritty, forms moderately hard ball when dry, stains, ribbons out on squeezing but the ribbon breaks easily.

Silty clay loam: Same as clay loam but very smooth, shows flaking on ribbon surface, similar to silt loam.

Very Fine Texture Clay: Very sticky, forms balls which on drying cannot be crushed by fingers, stains heavily, squeezes out at right moisture into long (2-5 cm) ribbons.

14. Determination of Penetration Resistance The readings taken with the penetrometer are called the cone index. The readings should be taken when the whole profile is at field capacity (approximately 24 hours after a soaking rain). If the soil is too wet (muddy), compaction could be underestimated because the soil acts as a liquid. If the soil is too dry, compaction could be overestimated because roots will be able to penetrate the soil when it dampens. The idea behind using the penetrometer at field capacity is that this is the best-case scenario for roots. During these periods, roots will be able to penetrate soil that has low penetration resistance. Penetration resistance will increase when the soil dries out, and root growth can then be expected to be limited. However, when the moisture content of the soil increases again, penetration resistance will decrease, and root growth will resume.

The penetrometer rod should be driven in the soil at a rate of approximately 1 inch per second. As one pushes the penetrometer into the soil, record the depth at which the 300 psi level is exceeded, using the gradients on the penetrometer rod. This level is the top of the compacted zone. Continue pressing the penetrometer down. Record the depth at which the penetration falls below 300 psi. This 33

is the bottom of the compacted zone. For each measuring point, there are two numbers: the top of the compaction zone and the bottom of the compaction zone. If penetration resistance never increases above 300 psi, you will have blanks in both spaces, indicating no severe root-limiting compaction. If the penetration resistance increases above 300 psi, but never falls below 300 psi, there is no bottom to the compaction zone.

Cone index should be measured respective to tillage relief, wheel tracks, plant rows, and other recognizable patterns in the field. For example, if one knows the areas of wheel traffic, take transects in and out of the track, and report them separately. If there are subsoiled zones in the field, measure penetration resistance in and out of the subsoiled zone. If there are planted rows, take measurements in and between the rows, and report them separately. Take separate readings for trafficked and non-trafficked areas.

The number of readings in a field depends on the accuracy one desires. As a first approximation, take some preliminary readings at a few places in the field to develop a sampling strategy. The cone index values are likely to be quite variable, so multiple readings are required per field. It is recommended to take one reading every 100 to 150 feet, or three to four readings per acre to develop a solid recommendation. This is a useful spacing if no recognizable patterns are present. If you recognize patterns, you may wish to increase the number of readings and report them separately as suggested above. It is extremely useful to compare the cone index values in the field with measurements in undisturbed areas such as fence rows.

15. Determination of Organic Carbon by volumetric method (Walkley and Black, 1934) Apparatus • Conical flask - 500 ml • Pipettes - 2, 10 and 20 ml • Burette - 50 ml Reagents • • • • •

Phosphoric acid – 85% Sodium fluoride solution – 2% Sulphuric acid – 96 % containing 1.25% Ag2SO4 Standard 0.1667M K2Cr2O7: Dissolve 49.04 g of K2Cr2O7 in water and dilute to 1 litre Standard 0.5M FeSO4 solution: Dissolve 140 g Ferrous Sulphate in 800 ml water, add 20 ml concentrated H2SO4 and make up the volume to 1 litre Diphenylamine indicator: Dissolve 0.5 g reagent grade diphenylamine in 20 ml water and 100 ml concentrated H2SO4.

Procedure 1.

2.

3.

4.

5.

6.

7.

Weigh 1.0 g of the prepared soil sample in 500 ml conical flask.

Add 10 ml of 0.1667M K2Cr2O7 solution and 20 ml concentrated H2SO4 containing Ag2SO4.

Mix thoroughly and allow the reaction to complete for 30 minutes.

Dilute the reaction mixture with 200 ml water and 10 ml H3PO4.

Add 10 ml of NaF solution and 2 ml of diphenylamine indicator.

Titrate the solution with standard 0.5M FeSO4 solution to a brilliant green colour.

A blank without sample is run simultaneously.

Calculation Percent organic Carbon (X) = [{10 (S - T) x 0.003}/(S)] x (100/Wt. of soil) Since one gram of soil is used, this equation simplifies to: {3 (S − T)}/S Where, 34

S = ml FeSO4 solution required for blank T = ml FeSO4 solution required for soil sample 3 = Eq W of C (weight of C is 12, valency is 4, hence Eq W is 12รท4 = 3.0) 0.003 = weight of C (1 000 ml 0.1667M K2Cr2O7 = 3 g C. Thus, 1 ml 0.1667M K2Cr2O7 = 0.003 g C) Organic Carbon recovery is estimated to be about 77%. Therefore, actual amount of organic carbon (Y) will be: 100 Percent value of organic carbon obtained x (100/77) Or Percentage value of organic carbon x 1.3 Percent Organic matter = Y x 1.724 (organic matter contains 58 % organic carbon, hence 100/58 = 1.724) Note: Published organic C to total organic matter conversion factor for surface soils vary from 1.724 to 2.0. A value of 1.724 is commonly used, although whenever possible the appropriate factor be determined experimentally for each type of soil.

35

Appendix IV Some other Information Guidelines for Sampling Depth S. No.

Soil sampling depth cm 5

Crop 1

Grasses and grasslands

2

Rice, finger millet, groundnut, pearl millet, small millets etc.(shallow rooted crops)

15

3

Cotton, sugarcane, banana, tapioca, vegetables etc. (deep rooted crops)

22

4

Perennial crops, plantations and orchard crops

Three soil samples at 30, 60 and 90 cm

General Recommended Dose of Micronutrient Fertilizers Micronutrient

Material and doses for application Soil application

Foliar spray

Zinc

Zinc sulphate (25 kg/ha)

0.5% zinc sulphate +0.25% lime

Iron

Ferrous sulphate (50 kg/ha)

1% ferrous sulphate + 0.5 % lime

Copper

Copper sulphate (10 kg/ha)

0.1% copper sulphate + 0.05% lime

Manganese

Manganese sulphate (10 kg/ha)

1% manganese sulphate + 0.25% lime

Boron

Borax (10 kg/ha)

0.2 % borax

Chemical Characteristics of Saline, Non-Saline Sodic and Saline Sodic Soils Soil

EC (dS/m)

ESP

pH

Saline

>4.0

<15

<8.5

Sodic(non-saline)

<4.0

>15

>8.5

Saline Sodic

>4.0

>15

<8.5

36

Literature Consulted B.K. Gugino, O.J. Idowu, R.R. Schindelbeck, H.M. van Es, D.W. Wolfe, B.N. Moebius-Clune, J.E. Thies, and G.S. Abawi. 2009. Soil Health Assessment Training Manual (Second Edition), College of Agriculture and Life Sciences, Cornell University. http://soilhealth.cals.cornell.edu/extension/manual/manual.pdf Dhyan Singh, Chhonkar, P.K and Dwivedi, B. S. (2005) Manual on Soil, Plant and water Analysis. 200 p. Westville Publishing House, New Delhi.

Diagnosing Soil Compaction Using a Penetrometer (soil compaction tester) http://extension.psu.edu/plants/crops/soilmanagement/soil-compaction/diagnosing-soil-compaction-using-a-penetrometer.

Lamond, R. and D.A. Whitney. 1992. Management of saline and sodic soils. Kansas State University, Department of Agronomy MF-1022.

Measuring soil depth. International Maize and Wheat Improvement Centre (CIMMYT). http://wheatdoctor.org/measuring-soil-depth Methods Manual, Soil Testing in India. (2011) Department of Agriculture & Cooperation, Ministry of Agriculture, Government of India, New Delhi. http://agricoop.nic.in/dacdivision/MMSOIL280311.pdf Tandon HLS (Ed). (2005). Methods of Analysis of Soils, Plants, waters, fertilizers and Organic Manures. Fertiliser Development and Consultation Organisation, New Delhi. India. Pp 204 +xii.

TNAU Agritech Portal. http://agritech.tnau.ac.in/agriculture/agri_soil_soilhealthcard.html.

USDA. Counting Earthworms Made Easy. http://www.ars.usda.gov/SP2UserFiles/Place/66120900/FactSheets/Schomberg/earthwormsonepageredited.pdf

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